The advent of powerful frequency-modulated infrared lasers brings into practical possibility the activation of chemical reactions by vibrational excitation. Because such reactions will involve systems with vibrational states out of thermal equilibrium, reactions may also be induced which are not normally observed. Furthermore, solid state reactions may be affected at cryogenic temperatures and equilibria may be displaced significantly by optical pumping. This paper examines the conditions necessary for infrared-laser-activated reactions. Appropriate experimental conditions are predicted.
One of the important applications of infrared lasers may well be the activation of highly selective chemical reactions and the study of their fundamental dynamics. Although initial success is likely to occur with gaseous reactions, liquid and solid reactions may follow. Isotopic separation may also be made highly selective by this technique.
Excitation by infrared lasers is fundamentally different than excitation by high energy lasers in the visible or u.v. range, the latter causing electronic transitions, usually with secondary energy transitions to the translational and vibrational degrees of freedom in a somewhat random fashion. Excitation by infrared places energy in the vibrational modes in a selective fashion, giving rise to the possibility of highly selective reactions.
We began to seriously consider the possibility of using infrared lasers for chemical activation over five years ago. However, at that time, the number of available infrared laser frequencies was severely limited and the techniques for modulation were not sufficiently advanced for our needs as we saw them then. Now a wide range of infrared laser frequencies can be generated and laser systems which can be tuned over wide frequency ranges are commercially available.
Practical modulation rates of these systems are still slow for this application, but it is conceivable that in the not-too-distant future frequency modulation over a wide range may be accomplished in nanosecond times. This will allow infrared cascading to be used to generate excited vibrational states resulting in larger populations than would be obtainable under thermal equilibrium conditions at thousands of degrees, under which conditions, of course, the simplest molecule would be torn apart.
Apparently the first experimental paper which describes laser infrared activation of a chemical reaction can be attributed to Borde et al who used a CO.sub.2 laser to excite and react SF.sub.6, C.sub.2 H.sub.4, C.sub.3 H.sub.6, and PH.sub.3..sup.(1) Mayer et al.sup.(2) used a continuous-wave hydrogen fluoride laser to successfully separate deuterium from hydrogen by specific activation of the reaction of methanol with bromine. Russian workers at the Lebedev Physics Institute, also known to be working in the field, have reported laser induced reactions with N.sub.2 F.sub.4, BCl.sub.3, SiH.sub.4, and SF.sub.6 which proceed at explosive rates..sup.(3)
We seriously considered the possibility of using infrared lasers for chemical activation in 1965. However, at that time, the number of available infrared laser frequencies was severely limited and the techniques for tuning were not sufficiently advanced for our needs as we saw them then, when we envisioned the need for a laser capable of being tuned over an appreciable range in a nanosecond. Now, in 1974, a wide range of infrared laser frequencies can be generated and laser systems which can be tuned over wide frequency ranges are commercially available. Practical modulation rates of these systems are still slow for this application, but it is expected that tuning over a wide enough frequency range may soon be accomplished in short enough times to minimize the effect of collisional process by cascading molecules into highly excited vibrational states in shorter than collision times. Such a capability will allow chemists and physicists to measure single vibrational and rotational relaxation processes definitively and to measure in detail the contribution of specific vibrational and rotational states to chemical reactions.
Infrared lasers already have been used in the study of vibrational relaxation times for some simple molecules..sup.(8,39,40) They have been used in conjunction with molecular beam experiments to elucidate the importance of vibrational energy in chemical reactions. .sup.(41) Some preliminary experiments have also been reported which verify that infrared lasers can markedly enhance chemical reactions even when competing collisional processes are important..sup.(1,2,42)
A number of recent papers report experiments and theory bearing on the general question of relaxation processes within the molecule, many of which must be considered as competitors to the actual reaction rate..sup.(43) Both intramolecular and intermolecular processes are important in the prediction of laser reaction enhancement. Considerable emphasis has been given to simple molecules in this regard, but little has been reported on vibrational relaxation processes of heavier molecules in the ground electronic state. Until this information is available, the complete potential of laser-enhanced reactions in activating specific chemical bonds cannot be completely assessed. Selective reactions for simple molecules, on the other hand, caused by energy enrichment of specific vibrational modes will most certainly produce products not normally observed under thermally equilibrated conditions.
Our approach has been to develop a mathematical formulation which, when used along with molecular dynamic measurements, can predict the rate enhancement caused by appropriately tuned lasers. The development follows the method used in transition state theory. It does not incorporate either the adiabatic condition nor does its usage depend upon the thermal equilibrium approximation, although it is derived from consideration of conditions of thermal equilibrium.
All molecules have characteristic vibrational frequencies. Their bonding is partly covalent and partly ionic, and all reactions which result in formation of new products involve breaking of bonds through extension of the distance between atoms within the molecule. The extension is a function of the vibrational excitation of the molecule, so the principles and techniques of this invention are applicable to all molecules. When the spectroscopy is known, excitation schemes can be developed and reaction conditions can be specified for production of specific compounds.
Efficient and easily employed conditions typically involve multiple excitation of a given molecule using one laser frequency, on a time scale that is short by comparison with the average time between molecular collisions.
An important objective of our latest activity is to select and experimentally measure the laser-enhanced reaction of a simple system which can be used to determine the predictive accuracy of the previously developed theory. The simplest systems involve molecules with only one degree of vibrational freedom where intramolecular relaxations do not occur. Hence, reactions involving diatomics are attractive.